The present invention generally relates to semiconductor devices and methods for their fabrication. More particularly, this invention relates to a micromachined microfluidic device and method that integrate a metallic packaging substrate to provide a fluid path through the device that is preferably free of organic materials. As nonlimiting examples, the microfluidic device can be configured as a Coriolis mass flow sensor, density sensor, specific gravity sensor, fuel cell concentration meter, chemical concentration sensor, temperature sensor, drug infusion device, fluid delivery device, gas delivery device, gas sensor, bio sensor, or medical sensor.
Processes for fabricating microelectromechanical system (MEMS) devices using silicon micromachining techniques are disclosed in commonly-assigned U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), and/or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. In the processes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628, plasma and wet etching, photolithography, and wafer bonding techniques are used to produce micromachined microfluidic devices comprising a micromachined tube supported above a surface of a substrate. The tube is fabricated to have an inlet, outlet, and fluid passage therebetween through which a fluid flows. The tube can be vibrated at resonance, by which the mass flow rate, density, and/or other properties or parameters of the fluid can be measured as it flows through the tube.
The micromachined tubes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628 can be fabricated from a semiconductor material, for example, doped or undoped silicon, and bonded to a substrate that may be formed of, for example, Pyrex, borofloat, quartz, or other glass-type inorganic amorphous solid, silicon, silicon-on-oxide (SOI), plastic, ceramic, or another material. Metal electrodes and runners used to carry electrical signals to and from the tube can be fabricated on the substrate. For purposes of conducting the fluid to and from the tube, the substrate may be further fabricated to have through-holes fluidically connected to the inlet and outlet of the fluid passage within the tube. For mass production, numerous micromachined tubes are preferably simultaneously fabricated in a semiconductor device wafer, which is then bonded to a substrate wafer, for example, by anodic, eutectic, solder, or fusion bonding. The resulting wafer stack then undergoes a dicing operation to singulate individual microfluidic device chips from the wafers.
In applications requiring protection of the micromachined tube, a capping die is preferably bonded to each microfluidic device to enclose and protect the tube. For the resonating tubes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628, the tube may be enclosed in a vacuum, which requires that the capping die is bonded (for example, anodically) to the device die to form a hermetic seal. For mass production, numerous capping dies can be simultaneously fabricated in a capping wafer, which is then bonded to the device-substrate wafer stack prior to the dicing operation such that dicing singulates individual capped microfluidic device chips from the capping-device-substrate wafer stack.
Device chips fabricated in the manner described above are typically attached to a package using an adhesive or solder. For example, a thin metal film can be deposited on the back of the device chip to provide a solderable surface, allowing the chip to be soldered to a package in a subsequent packaging step, for example, an IC packaging process using a packaging material such as a plastic or metal (for example, KOVAR®). Anodic bonding of individual device chips to a metal substrate has also been proposed, as disclosed in Briand et al., “Metal to glass anodic bonding for Microsystems packaging,” Transducers 2003, Vol. 2, 4C2.2, pp. 1824-1827 (2003).
The use of a metallic instead of plastic package can be advantageous if packaging stresses are of concern, including those attributable to differing coefficients of thermal expansion (CTE), or if the fluid operated on by the microfluidic device, for example, a low or high pH liquid, a high temperature or corrosive gas or biofluid, solvent, etc., is incompatible with common plastic materials due to the risk of corrosion, contamination, or biocompatibility issues. For example, the presence of plastic and other organic materials in the package or the fluid flow path can lead to contamination, out-gassing, melting or decomposition of the organic materials.